Add a new PSR-17 factory to Psr17FactoryProvider

[solventt]

There is curious performance benchmark of the PSR-7 and PSR-17 implementations (Guzzle, HttpSoft, Laminas, Nyholm, Slim) - https://github.com/devanych/psr-http-benchmark.

The fastest is HttpSoft, the slowest - Slim.

So I've added HttpSoftPsr17Factory to the Slim's factory set to give users the possibility to use the HttpSoft implementation of the PSR-7.

What was generally done:

1) Adding HttpSoftPsr17Factory::class to the $factories property of the Slim\Factory\Psr17\Psr17FactoryProvider class

protected static $factories = [
    SlimPsr17Factory::class,
    HttpSoftPsr17Factory::class,
    NyholmPsr17Factory::class,
    LaminasDiactorosPsr17Factory::class,
    GuzzlePsr17Factory::class,
];

I placed the class for the second position because it's the fastest implementation of the PSR-7 according to the benchmark. But the Slim's implementation remains in first place because I think it's normal to give priority to packages from the Slim's ecosystem.

2) Adding the new HttpSoftPsr17Factory class to the Slim/Factory/Psr17 folder

3) Adding the httpsoft/http-message and httpsoft/http-server-request packages to the composer.json file (--dev section)

4) Adding [HttpSoftPsr17Factory::class, HttpSoftServerRequest::class] to the provideImplementations() method in the ServerRequestCreatorFactoryTest class

5) Adding [HttpSoftPsr17Factory::class, HttpSoftResponseFactory::class] to the provideImplementations() method in the AppFactoryTest class

6) Editing README.md (added a mention of the HttpSoft PSR-7 implementation along with other implementations)

About tests

• There were: 420 tests, 779 assertions
• Became: 422 tests, 781 assertions

[coveralls]

Coverage remained the same at 100.0% when pulling fe7cab8 on solventt:new-psr17-factory into e208383 on slimphp:4.x.

[l0gicgate]

This is a great contribution, I was not aware that our implementation was this slow. We should probably do some digging since the benchmarking code just seems to be instantiating the objects.

I also wasn't aware of the HttpSoft implementation!

We won't be able to merge this until we bump PHP lowest version support to 7.4 in about a month unfortunately

[l0gicgate]

We can deprecate PHP 7.3 as of Today @solventt if you would like to add that to this PR

[solventt]

@l0gicgate The conflict in composer.json has been resolved

[l0gicgate]

Thank you for this contribution @solventt